1. Field of the Invention
The present invention relates to a spin valve sensor having a free layer stabilized by ferromagnetic and sense current fields and, more particularly, to a ferromagnetic coupling field that has a horizontal component for strengthening longitudinal biasing of the free layer and a sense current field that is in the same direction as a vertical component of the ferromagnetic coupling field for strengthening transverse biasing of the free layer.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly mounted on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent the ABS to cause the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head may include a coil layer embedded in first, second and third insulation layers (insulation stack) with the insulation stack, in turn, being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a nonmagnetic gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field into the pole pieces that fringes across the gap between the pole pieces at the ABS. The fringe field or the lack thereof writes information in tracks on moving media, such as in circular tracks on a rotating disk.
In recent read heads a spin valve sensor is employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer, and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is typically pinned perpendicular to the air bearing surface (ABS) of the head and the magnetic moment of the free layer is typically oriented parallel to the ABS, but free to rotate in response to external magnetic fields from the rotating disk. The magnetization of the pinned layer is pinned by exchange coupling with an antiferromagnetic (AFM) pinning layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layers are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos xcex8, where xcex8 is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor resistance changes of the sensor cause potential changes that are detected and processed as playback signals by the processing circuitry.
The spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. MR coefficient is dr/R were dr is the change in resistance of the spin valve sensor and R is the resistance of the spin valve sensor before the change. A spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. A spin valve is also known as a top or bottom spin valve depending upon whether the AFM pinning layer is at the top (AFM layer formed after the free layer) or at the bottom (AFM layer formed before the free layer). A pinning AFM layer in a bottom spin valve is typically made of nickel oxide (NiO). The spin valve sensor is located between first and second nonmagnetic electrically insulative read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
The signal performance of the spin valve sensor is poor unless the free layer is magnetically stabilized. The free layer is magnetically stabilized when its magnetic spins are in a single magnetic domain state. The free layer is not magnetically stabilized when the magnetic spins are oriented in multiple magnetic domains. Magnetic domains have domain walls which move when the free layer is subjected to an applied field. This movement causes unpredictable magnetic fields within the free layer which is superimposed upon the read signal from the rotating disk. Accordingly, the desired read signal is contaminated by internal magnetic signals within the free layer due to movement of the domain walls.
In order to overcome the instability of the free layer hard biasing layers are typically employed at side edges of the free layer for longitudinally magnetically biasing the free layer parallel to the ABS. First and second hard biasing layers may make contiguous junctions with first and second side edges of the spin valve sensor or first and second hard bias layers may overlap first and second layer portions of the spin valve sensor in passive regions of the sensor. As stated hereinabove, the spin valve sensor is located between first and second read gap layers. It is important that these read gap layers be extremely thin in order to promote linear bit read density of the read head. Linear bit density is determined by the distance between the first and second shield layers and is the length of the signal along the circular track of the rotating disk that the read head is capable of sensing. This length is reduced when the thicknesses of the first and second read gap layers is reduced thereby increasing the number of magnetic bits that the read head is capable of reading along the circular track which is referred to in the art as the aforementioned linear bit read density of the read head.
Unfortunately, when the first and second read gap layers are thin the longitudinal biasing field from the hard biasing layers quickly decays due to leakage to the first and second shield layers. While longitudinal biasing typically exists at the first and second side edges of the spin valve sensor, it progressively decays toward the center of the spin valve sensor where it may drop to zero. Accordingly, there is no or little longitudinal biasing at a center portion of the spin valve sensor. It has been found that a mere increase in the strength of the hard biasing layers does not overcome this problem. While the problem can be reduced by increasing the thickness of the first and second gap layers, this will reduce the linear bit read density of the read head which equates to reducing the magnetic storage capability of the magnetic disk drive. Accordingly, there is a strong-felt need for overcoming the magnetic instability of the free layer while maintaining high linear read bit density.
The present invention provides supplemental longitudinal biasing of the free layer for improving magnetic stability of the free layer in high linear bit read density read heads. This is accomplished by orienting the magnetic moment of the pinned layer at a slight angle xcex8 to a normal to the air bearing surface. The angled magnetic moment of the pinned layer, in turn, exerts a correspondingly angled ferromagnetic coupling field on the free layer. The angled ferromagnetic coupling field on the free layer has a small horizontal ferromagnetic coupling field which is equal to HFC sin xcex8. It is important that the pinned layer be pinned in a direction so that the horizontal component of the ferromagnetic coupling field is in the same direction as the hard biasing field on the free layer. It is also important that the angle xcex8 be small, less than 10xc2x0 and preferably less than 5xc2x0, since this angle reduces the vertical component of the magnetic moment of the pinned layer which is necessary for obtaining a desired read signal response. The horizontal component of the ferromagnetic coupling field on the free layer is highly effective in overcoming the decay problem of the hard biasing field since the horizontal component is constant throughout the entire width of the free layer.
Another aspect of the invention is improving the magnetic stability of the free layer with proper orientation of the sense current field. When the aforementioned first and second lead layers conduct a sense current through the spin valve sensor the sense current through the layers other than the free layer cause a net sense current field on the free layer which is transverse to the air bearing surface. The sense current field is zero at the top and bottom edges of the free layer and increases to a maximum at the center of the free layer between its top and bottom edges. The top and bottom edges of the sensor define the stripe height of the sensor. The ferromagnetic coupling field on the free layer is constant between its top and bottom edges. When the sense current field is oriented opposite to the direction of the ferromagnetic coupling field a net transverse field exists at the first and second side edges of the free layer, but can decrease to zero or go to an opposite polarity at the center of the stripe height. I have found that by conducting the sense current in an opposite direction that the net sense current field on the free layer is in the same direction as the ferromagnetic coupling field on the free layer so that the ferromagnetic coupling field and the net sense current field are additive. The additive effect of the ferromagnetic coupling field and the sense current field maintains a transverse biasing field on the free layer that increases from the top and bottom edges of the free layer toward the center of the stripe height for magnetically stabilizing the free layer.
In a preferred embodiment both of the foregoing aspects of the invention are combined to magnetically stabilize the free layer. Accordingly, the magnetic moment of the pinned layer is angled to provide a horizontal component of the ferromagnetic coupling field on the free layer to support the hard biasing of the free layer in combination with a sense current field that is appropriately directed to support the vertical component of the ferromagnetic coupling field on the free layer.
An object of the present invention is to magnetically stabilize a free layer of a spin valve sensor in high bit density read heads.
Another object is to provide a supplemental magnetic field or magnetic fields on the free layer in addition to a longitudinal biasing field for magnetically stabilizing the free layer.
A further object is to maintain the free layer in a single magnetic domain state in a magnetic read head that has minimally thick first and second read gap layers.
Another object is to employ a ferromagnetic coupling field on the free layer which has a component in the same direction as a magnetic field from hard biasing layers for magnetically stabilizing the free layer.
Still a further object is to transversely bias the free layer perpendicular to the air bearing surface for supplementing a ferromagnetic coupling field on the free layer so as to improve the magnetic stability of the free layer.
Still a further object is to orient a horizontal component of a ferromagnetic coupling field on the free layer in the same direction as longitudinal biasing by first and second hard biasing layers and employ a sense current field on the free layer which is in the same direction as a vertical component of the ferromagnetic coupling field for enhancing magnetic stability of the free layer.
Other objects and advantages of the invention will become apparent upon reading the following description taken together with the accompanying drawings.